## Advice for New Graduates During Downturn of Oil and Gas Industry

Obviously this isn’t the best time to graduate in Geophysics, Geology or Engineering if you have a plan to make a career in oil & gas at this time 2016. However, it isn’t the first time that such precipitous fall in the oil prices has taken place, and many people in the industry today graduated when the prices of oil previously hit low levels, quite similar to the present situation. So what are the things that new graduates should do in order to position themselves for the time when oil prices finally begins rising again and “recovery” starts in the oil industry.

## Determine Compressibility Factor with Present of CO2 and H2S

This example will demonstrate how to determine the compressibility factor (z) for gas with CO2 and H2S. As it is described in the article,Determine Compressibility of Gases , it states that gas with CO2 and H2S must be corrected. This is similar to a normal method for determining z-factor but it is required some correction. Please follow the steps below;

Gas component is shown in Table 1

Table 1 – Gas Component

Reservoir pressure = 6,000 psia

Reservoir temperature = 190 F Continue reading

## Example of Real Gas Calculation

This example will demonstrate how to calculate the compressibility of real gas in order to determine gas density and specific gravity at a specific condition.

Calculate the following based on the given condition:

1) Density of this gas under the reservoir conditions of 7,500psia and 220ºF,

2) Specific gravity of the gas.

Gas component is shown in Table 1

Table 1 – Gas Component

Average density of air = 28.96 lb/cu-ft

## Solution

• Determine critical pressure and temperature of gas mixtures using Kay’s rule

Table 2 – Critical Pressure and Temperature

Note: critical pressure and temperature can be found from this link – https://www.drillingformulas.com/determine-compressibility-of-gases/

Pc’ = Σyipci = 660.5 psia

Tc’ = ΣyiTci = -46.2 F = -46.2 +460 F = 413.8 R

Table 3 – Pc’ and Tr’ by Kay’s Rule

• Calculate Tr and Pr

Tr = T ÷Tc

Tr = (220+460) ÷ (-46.2+460)

Tr = 1.64

Note: temperature must be in Rankin.

Rankin = Fahrenheit + 460

For the critical temperature calculation, it can be converted the critical temperature from F to R before calculating Tc’. This will still give the same result.

Pr = P ÷ Pc

Pr = 7500 ÷ 660.5 = 11.4

• Read the compressibility factor (z) from the chart.

z = 1.22

Figure 1-z-factor from the Standing and Katz Chart

• Calculate average molar mass

Average Molar Mass = Σyi×Mi = 22.1 lb

Table 4 – Average Molar Mass of Gas

• Calculate density of gas from the equation below;

Gas Density = 18.6 lb/cu-ft

• Calculate gas specific gravity from the equation below;

SG = Gas Density ÷ Air Density

SG = 18.6 ÷ 28.96

SG = 0.64

Summary:

Gas Density = 18.6 lb/cu-ft

SG = 0.64

We wish that this example will help you understand to determine z-factor and use it to calculate any related information.

References

Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.

## Determine Compressibility of Gases

This article will demonstrate how to determine gas compressibility by using simplified equation of state.

At a low pressure condition, gas similarly behaves like ideal gas and the equation for the ideal gas law is listed below;

# PV = nRT

Where;

P = pressure

V = gas volume

T = absolute temperature

n = number of moles of gas

R = gas constant

Gas constant (R) is different because it depends on the unit system used in the calculation. The R in the different unit system is demonstrated in the below table.

Table 1 – Gas Constant (R) at Different Unit

## Material Balance for Gas Reservoir

For gas reservoirs, the material balance concept can be applied to determine gas in place and expected gas reservoir reserve.

Gas Production = Expansion of Free Gas In Reservoir

Assumption:

• Dry gas reservoir
• No external energy support like water drive.

Where;

Gp = gas production (std cu-ft)

Bg = gas formation volume factor (res cu-ft/std cu-ft)

G = gas in place (std cu-ft)

Bgi = initial formation volume factor (res cu-ft/std cu-ft)

Gas formation volume factor (Bg)